U.S. Pat. Nos. 2,515,936, 2,575,943, and 2,651,145 describe the manufacture of transparent and thermally opacifiable photosensitive glasses.
U.S. Pat. No. 2,515,936 discloses the preparation of transparent, colored glasses having base compositions within the alkali metal-alkaline earth metal-silica field containing silver and CeO.sub.2 as a sensitizing agent. The glass is exposed to such shortwave radiations as ultra-violet radiations, X-radiations, and radioactive radiations and thereafter heated to about 470.degree.-500.degree. C. Colloidal particles of silver are developed which imparted color to the glass.
U.S. Pat. No. 2,575,943 describes thermally-opacifiable, photosensitive glasses having base compositions within the Na.sub.2 O-Al.sub.2 O.sub.3 SiO.sub.2 -F system containing gold and CeO.sub.2 as a sensitizing agent. The glass is subjected to shortwave radiations and then heated to a temperature between about 100.degree. C. below the softening point of the glass to about 50.degree. C. above the softening point. That treatment gives rise to the development of colloidal gold nuclei. The nucleated glass is cooled to below 520.degree. C. to effect the formation of NaF nuclei on the colloidal gold particles, and thereafter the glass is heated to about 520.degree.-570.degree. C. to cause the NaF crystallites to grow to sufficient size to scatter visible light. The opaque area of the glass was commonly white in color although the occurrence of a pink tint was mentioned.
U.S. Pat. No. 2,651,145 was concerned with thermally-opacifiable, photosensitive glasses having base compositions within the Na.sub.2 O-Al.sub.2 O.sub.3 -SiO.sub.2 -F field containing silver and CeO.sub.2. The glass was exposed to shortwave radiations and subsequently heated to a temperature between about 150.degree. C. below the softening point of the glass to about 50.degree. C. thereabove. Colloidal silver nuclei were developed thereby. The nucleated glass was cooled below 500.degree. C. to form NaF nuclei on the metallic silver particles. The glass was thereafter heated to a temperature no lower than 100.degree. C. below the softening point of the glass to cause the NaF nuclei to grow to a sufficient size to scatter visible light. Differently colored opacified articles could be produced by varying the shortwave exposure time and the time and/or temperature of the first heating step.
Whereas those patents indicated that colored transparent and opacified (opal) glasses could be produced from glasses containing photosensitive metals, control of the colors produced was extremely difficult and there was the desire to be able to obtain a full spectrum of colors in a single glass composition.
These circumstances led to U.S. application Ser. No. 646,259, now Pat. No. 4,017,318, filed Jan. 2, 1976 by J. E. Pierson and S. D. Stookey, which discloses the preparation of photosensitive colored glasses or polychromatic glasses, as they have been variously denominated. That application describes two general methods for the production of such glasses, each method comprehending a series of irradiation and heat treating steps. A wide range of base glass compositions is operable in the invention but, as essential ingredients, each must include silver, an alkali metal oxide which is preferably Na.sub.2 O, fluoride, and at least one halide selected from the group of chloride, bromide, and iodide. The glass articles are irradiated with high energy or actinic radiations selected from the group of high velocity electrons, X-radiations, and ultra-violet radiations having wavelengths within the range of about 2800A-3500A. The heat treatments contemplate exposing the glass articles to temperatures between about the transformation range up to about the softening point thereof. Where ultra-violet radiation comprises the effective actinic radiation, CeO.sub.2 is a required constituent of the glass composition.
In the first general method disclosed therein, the glass article is initially exposed to high energy or actinic radiations, which exposure causes the development of a latent image in the glass. The time and intensity of this first exposure serves to determine the final color which will be produced in the glass. Subsequently, the exposed glass article is subjected to a heat treatment which effects the precipitation of colloidal silver particles in situ to function as nuclei. Where a colored transparent glass article is sought, this heat treatment will be conducted for only so long as to cause the precipitation of colloidal silver and to occasion the growth thereon of extremely small microcrystals of alkali metal fluoride-silver halide, e.g., NaF + (AgCl and/or AgBr and/or AgI). Where a colored opal glass article is desired, this heat treatment will be extended for a length of time adequate to not only effect the precipitation of colloidal silver nuclei, but also to cause the growth of said microcrystals on the silver nuclei to a large enough size to scatter light. Thus, the opacity is developed in the exposed portions of the glass article with the unexposed portions, if any, remaining clear.
The so-nucleated glass article is thereafter cooled to a temperature at least 25.degree. C. below the strain point of the glass, conveniently to ambient or room temperature, and then again exposed to high energy or actinic radiation. This second exposure serves to intensify the color which will subsequently be developed, the hue of which was determined previously through the first exposure. Finally, the glass article is again heated to a temperature at least about the transformation range of the glass and up to about the softening point thereof to produce the desired color in the glass. The inventors hypothesized that this final heat treatment caused the precipitation of submicroscopic particles of silver, either as discrete colloidal particles and/or deposited on the surface and/or deposited within the alkali metal fluoride-silver halide microcrystals.
Whereas the mechanism of the color phenomenon is not totally understood, it is believed to be a function of the quantity of silver precipitated and the geometry thereof, as well as, perhaps, the refractive index of the crystals. Furthermore, inasmuch as the colors can be secured with very low contents of silver and illustrate characteristics akin to interference colors, it has been conjectured that at least one of the three following circumstances is present: (1) discrete colloidal particles of silver less than about 200A in the smallest dimension; (2) metallic silver deposited within the alkali metal fluoride-silver halide microcrystals, the silver-containing portion of the microcrystals being less than about 200A in the smallest dimension; and (3) metallic silver deposited upon the surface of said microcrystals, the silver-coated portion of the microcrystals being less than about 200A in the smallest dimension.
The inventors then observed that the heat treatment after each exposure to high energy or actinic radiation could desirably take the form of a series of heatings and coolings instead of a single treatment as set out above. Such additional treatments do not alter the color developed but can intensify the color produced.
The second fundamental method for producing colored glasses outlined in the above application disclosed the preparation of glasses exhibiting a single color, this color being capable of being varied over the entire range of the visible spectrum. Glass batch compositions similar to those described above with respect to the first method are also useful here, but such will also contain a thermoreducing agent, desirably SnO and/or Sb.sub.2 O.sub.3. The method involves four basic steps:
First, a glass-forming batch of the desired composition and containing a thermoreducing agent is melted and formed into a glass article;
Second, the glass article is subjected to a heat treatment at temperatures between about the transformation range of the glass and the softening point thereof for a period of time sufficient to induce reduction of part of the silver content to colloidal silver particles, thereby acting as nuclei, and to cause the growth of alkali metal fluoride-silver halide microcrystals thereupon;
Third, the nucleated glass article is cooled to at least 25.degree. C. below the strain point of the glass and exposed to high energy or actinic radiation;
Fourth, the glass article is reheated to a temperature between about the transformation range of the glass and the softening point thereof to effect the precipitation of submicroscopic particles of silver, either as discrete colloidal particles and/or deposited on the surface and/or deposited within the alkali metal fluoride-silver halide microcrystals.
The color to be developed within the glass is determined by the extent of thermal reduction in the second step, and the intensity of the color is affected by the parameters of the third and fourth steps. The use of a series of heat treatments, either after the initial heat treatment to effect thermoreduction or after the exposure to high energy or actinic radiation, can be useful in intensifying the final color produced. Thus, whereas the reaction mechanism at work is not completely understood, it appears that two or more consecutive heat treatments at temperatures between about the transformation range and the softening point of the glass, separated by cooling to below the transformation range, lead to a more vivid final color than a single heat treatment of equal or longer duration.
The application also pointed out that the sequence of colors produced seemed to be similar with both general methods. Hence, in the first method wherein two exposures to high energy or actinic radiation were involved, the briefest initial exposure provided for the development of a green color, followed by blue, violet, red, orange, and yellow as the exposure time and/or intensity is increased. In like manner, the least amount of thermal reduction will result in a green glass and the most will yield a yellow glass.
U.S. application Ser. No. 778,160, filed concurrently herewith by Joseph Ference, discloses an improvement upon the method described in application Ser. No. 646,259. That improvement comprises combining the exposure to high energy or actinic radiation and heat treatment into a single step. Thus, the exposure is conducted while the glass article is at a temperature between about 200.degree.-410.degree. C. The invention is operable over the broad range of glass composition recited in application Ser. No. 646,259, and is particularly advantageous in producing a similar product while both reducing the treatment time required to achieve a colored glass article and improving the intensity of the color developed.